The present invention involves an improved bushing apparatus for making fibers, particularly glass fibers and an improved method of making and using glass fiberizing bushings.
In the manufacture of continuous fibers from a molten material like molten glass, the molten material is often generated by a tank furnace and distributed to a plurality of fiberizing bushings via one or more channels and one or more bushing legs connected to the channel(s). Each bushing leg comes off the channel at about 90 degrees and contains a plurality of bushings that are spaced apart.
Precious metal bushings made from alloys of platinum and rhodium and used for making continuous glass fibers are well known, having been in use for more than 50 years. Many types of bushings exist for converting molten glass into continuous glass fiber and products. Typical types of bushings are shown in U.S. Pat. Nos. 3,512,948, 4,155,732, 4,272,271 and 4,285,712, the disclosures of which are hereby incorporated by reference. All the bushings shown in these patents teach the use of a perforated plate or screen, welded to the endwalls and sidewalls at some distance above a tip plate containing hundreds or thousands of nozzles or tips where molten glass first emerges from the bushing and is converted to continuous glass fibers by cooling and drawing, attenuating, in a known manner. Some of these patents teach various means of reinforcing the tip or orifice plate through which the molten material flows to form fibers. The bushings are electrically heated by their own resistance and are box-like, open on the top and comprise an orifice plate containing many orifices or tips welded therein, side walls, end walls, terminals on the end walls for connecting electrical cables, a top flange for contacting the underneath side of a forehearth, and usually a perforated plate or screen parallel with, but mounted above, the orifice plate. Usually the bushings are made by cutting the parts from alloy of desired thickness and welding the parts together with similar alloy, but a part or all of the bushing can be made by casting as is known.
These bushings work well as long as the tip plate remains fairly flat. At the high temperature at which these bushings operate, usually above 2000 degrees F., the platinum-rhodium alloy tip plate creeps (stretches permanently under load) and sags with time until the amount of sag becomes so great that it is no longer possible to cool the tips and the molten glass forming the fibers below the tips sufficiently uniformly, at which time the break rate of fibers becomes very high and uneconomical. Very soon after the first fiber breaks out on a bushing making E glass, the most common glass used to make continuous fiber products, all of the fibers break out in a chain reaction due to one or more beads of molten glass falling from the broken out tip into the array of fibers from the other tips.
It is common practice to support tip plates from below the bushing with linear supports between rows of tips at one or more locations. These linear supports usually run down the length of the tip plate and must be a refractory and electrically insulating material or separated from the bushing with such a material.
In spite of the supports, the portions of the tip plate between the supports still sags and causes the temperature profile of the tip plate to be non-uniform because it causes tips to be different distances away from the cooling means commonly used such as finned cooling tubes or cooling fins. When this happens, portions tip plate, and tips therein, that have sagged the least run considerably hotter than the tips in the portions of the tip plate that have sagged the most because of the distance between the tips and the cooling means.
Since the heat transfer at these very hot temperatures is very dependent on the distance between the hot surface and a cold surface, distance variations are very critical to keeping the molten glass within an acceptable temperature range and viscosity to maintain fiberization, particularly in larger bushings containing 1600 or more tips or orifices. When tips, due to sag, get very close or touch a cooling tube or fin, the cooling rate causes the molten glass to cool excessively resulting in a viscosity so high that the fiber either breaks in the attenuation zone below the tip or in a much smaller diameter fiber which is broken more often due to defects in the molten glass that cause a higher break rate as the fiber diameter becomes smaller. Also, even if the cooling in the vertically lowest tips is not so great that too frequent breaks occur, nevertheless the fiber diameters generated from those tips are out of specification to the low side making the fiber diameter distribution from the entire bushing undesirable or unacceptable to the customers.
The cooling means can be adjusted some to compensate for sagging tip plates and tip ends being at a different level in different portions of the bushing, but this requires great skill in very uncomfortably hot conditions. Because of this and the proximity to surfaces that will produce severe burns, adjustments are not made as often as they should be. Also, frequently the bushing tips are damaged during such adjustments causing excessive fiber breaks from those tips and/or necessitating that those tips be crimped shut, reducing the productivity of the bushing. When this becomes uneconomical, the bushing must be shut down, removed and a new bushing installed. Often, for some time before the bushing is changed out, the bushing runs at a significantly lower productivity than a new bushing that has not yet sagged significantly.
It is very expensive to replace a bushing. As a result of development, large bushings of 3000 or more tips, such as 4000 or more tips, are now used for making the majority of the tonnage of continuous glass fiber produced today. Such bushings cost several thousands of dollars to fabricate even though the precious metal from removed bushings is recycled. A typical large producer will have more than 200 of such bushings in production. In addition to the fabrication cost of a new bushing, the labor, lost production and other damage to the forehearth and adjacent bushings adds substantial additional costs to the total cost of removing an inoperative bushing and installing a new bushing. These costs typically amount to more than $5,000 per bushing change.
It would be very advantageous to find a way to prevent, or substantially retard the rate of, tip plate sagging, thus extending the production time and average productivity or fiberizing efficiency of each bushing. The fiberizing efficiency is the percentage of time that the bushing is producing good fiber product compared to the total time the bushing is operating e.g. an operating efficiency of 94 percent means that the bushing is producing good fiber product 94 percent of the total time that the bushing is at operating temperature.
It is an object of the present invention to significantly reduce the rate of sag of tip or orifice plates on precious metal bushings improving the temperature consistency of the molten material exiting the tips or orifices in the tip or orifice plate of the bushing at over 1800, preferably 1900 degrees F. during the life of the bushing and to increase the average fiberizing efficiency over the life of the bushing and to increase the life of the tip or orifice plates of the bushing.
Another object of the invention is a process of making fiber from a molten material by flowing the molten material through holes and/or nozzles in an orifice plate of an electrically heated bushing that has a tip or orifice plate that resists sagging better than prior art bushings and that results in a better average efficiency during the normal bushing life than has been experienced with prior art processes.
Another object of the invention is a bushing used for, and a method of, making continuous glass fiber products that result in glass fibers having a lower variation of fiber diameters in the array of fibers coming from the bushing.
Another object of the invention is a process of making a precious metal bushing for producing continuous fiber products by installing a novel tip plate reinforcing structure.
These objects are accomplished in the fiberizing bushings of the present invention by using a tip plate or orifice plate reinforced with internal reinforcing members, internal supports, welded to the top surface of the tip or orifice plate along at least portions of the lower edge(s) of the supports in the fiberizing bushings. Preferably, one end of each of four of the internal reinforcing members, supports, being welded to an inside corner of the bushing with the other end of each reinforcing member being welded to an opposite sidewall of the bushing. The supports intersect or cross one another and form one angle of significantly less than 90 degrees with at least one sidewall of the bushing. This angle can be 60 degrees +/xe2x88x9220 degrees, but is preferably about 60 degrees +/xe2x88x9215 degrees and most preferably 60 degrees +/xe2x88x9210 degrees and 60 degrees +/xe2x88x925 degrees is particularly effective. The bushing tip or orifice plate is round, oval or generally oval, square or preferably rectangular in that portion containing the internal reinforcing supports, and preferably a bushing screen contacts the reinforcing members, at least down the center portion of at least most of the length of the bushing, the width of the screen contact portion to be at least about 25 percent of the width of the bushing in the portion of the bushing containing internal reinforcement members.
The length of the bushing is the dimension of greatest magnitude in that portion of the bushing containing the internal reinforcing members and the width of the bushing is the dimension of the bushing perpendicular with the length, parallel with the orifice or tip plate and in that portion of the bushing containing the reinforcing members, the width not being in the direction of the thickness of the tip plate. The screen can contact the entire top length of all reinforcing members or internal supports from sidewall to sidewall and from end wall to end wall, but preferably contacts the top edges of only the central portion of the many of the internal supports from one endwall to the other endwall. The internal supports intersect each other and preferably these intersections are located directly above the areas where the orifice plate or tip plate is supported by linear external support bars, usually of a ceramic material. Typically each internal support is spaced within four inches of another, preferably parallel, internal support, preferably within about three inches and most preferably within about 2 inches or less.
Preferably the fiberizing bushing of the present invention also has external supports contacting an external surface of the tip plate or orifice plate, and preferably these external supports run down the length or most of the length of the tip plate or orifice plate. These external supports are known in the art and can be of the type disclosed in U.S. Pat. No. 4,356,016 or can be sintered high alumina shapes of the type shown in FIGS. 2 and 2A, preferably having an alumina content of at least about 72 percent with the remainder being mainly silica or magnesia.
The invention also comprises a method of making fiber from a molten material, preferably molten glass, by flowing the molten material into an electrically heated, precious metal alloy fiberizing bushing having, at least one generally vertical side wall, an orifice/tip plate having holes therein and internal reinforcing members thereon, and a perforated plate mounted in the bushing above the orifice/tip plate, and causing the molten material to flow through the holes whereby fibers are formed below the bushing in a continuous manner, the improvement comprising reinforcing members being welded to the tip or orifice plate along at least portions of the lower edge(s) of at least most, and preferably all, of the internal reinforcing members or supports, one end of each of four of the internal reinforcing members being welded to an inside corner of the bushing with the other end being welded to an opposite sidewall of the bushing. The internal reinforcing members form an angle with at least one sidewall that is significantly less than 90 degrees and preferably intersect each other. This angle can be 60 degrees +/xe2x88x9220 degrees, but is preferably about 60 degrees +/xe2x88x9215 degrees and most preferably 60 degrees +/xe2x88x9210 degrees and 60 degrees +/xe2x88x925 degrees is particularly effective. Preferably these intersections are located directly above the areas where the orifice plate or tip plate is supported by linear external support bars, usually of a ceramic material.
The bushing tip or orifice plate is round, oval or generally oval, square or rectangular in that portion containing the reinforcing members, preferably rectangular, and preferably a bushing screen contacts the reinforcing members, at least down the center portion of at least most of the length of the bushing. Normally the bushing would have two sidewalls and two endwalls, but two or all of these can be combined into a single wall, such as where the bushing is circular, oval, etc.
When the word xe2x80x9caboutxe2x80x9d is used herein it is meant that the amount or condition it modifies can vary some beyond that so long as the advantages of the invention are realized. Practically, there is rarely the time or resources available to very precisely determine the limits of all the parameters of ones invention because to do would require an effort far greater than can be justified at the time the invention is being developed to a commercial reality. The skilled artisan understands this and expects that the disclosed results of the invention might extend, at least somewhat, beyond one or more of the limits disclosed. Later, having the benefit of the inventors disclosure and understanding the inventive concept and embodiments disclosed including the best mode known to the inventor, the inventor and others can, without inventive effort, explore beyond the limits disclosed to determine if the invention is realized beyond those limits and, when embodiments are found to be without unexpected characteristics, those embodiments are within the meaning of the term about as used herein. It is not difficult for the skilled artisan or others to determine whether such an embodiment is either as might be expected or, because of either a break in the continuity of results or one or more features that are significantly better than reported by the inventor, is surprising and thus an unobvious teaching leading to a further advance in the art.